This invention is directed to a ion implantation equipment which is capable of automatically forming an impurity layer having a uniform distribution of impurity concentration in the direction of depth of a semiconductor wafer.
In producing patterns of integrated circuits, it is required, as is generally known, to make the patterns extremely fine. For this purpose, it is necessary to control minutely the distribution of impurity concentration not only in the lateral direction but also in the vertical direction of the semiconductor wafer.
Hitherto, formation of impurity layers has been effected by means of thermal diffusion of impurity material. Thermal diffusion being a statistical phenomenon, control of the profile of impurity layers was not effectively achieved.
Recently, technology has been developed to implant ions of an impurity material into the wafer. In order to obtain a desired profile of the impurity layer by ion implantation, treatment of the substrate at relatively low temperature or laser annealing has been proposed to prevent thermal diffusion of the implanted ions. It is therefore important that the desired profile is produced by ion implantation prior to such treatment.
Ion implanting an impurity material into the surface of a silicon semiconductor wafer 2 two dimensionally in the X- and Y-directions is shown in FIG. 1. If the acceleration voltage or energy of an ion beam of which trace is shown by the dotted line 4 in FIG. 1 is constant, shallow impurity layers are formed which have the maximum impurity concentration at a predetermined depth from the surface of wafer 2. In order to form impurity layers of constant concentration having the desired thickness in the direction of depth as shown in FIG. 2, the acceleration voltage or energy E of the ion beam must be successively altered from E.sub.1 to E.sub.n to repeat the process of ion implantation n times. Conventional equipments for ion implantation are provided with thumb nuts or finger screws for manually adjusting the acceleration voltage or energy E by the operator, but it is an extremely complicated task for the operator to repeat n times the manipulation of ion implanting at acceleration voltage E.sub.1 for a predetermined length of time, at voltage E.sub.2 for another length of time and so forth. If each difference of energy increase represented by E.sub.2 -E.sub.1 and so forth is made larger, that is if n is made smaller, the abovedescribed manual operation may be rendered simpler. However, if n is made smaller, the change of the impurity concentration in the direction of depth of the wafer becomes inevitably large.